Effect of Focal Spot on Resolution (Contact Radiograph)
The radiograph shown above is of a foot phantom together with a line pair phantom. The image was obtained using a relatively low voltage (50 kV), since the object thickness is relatively low and is easy to penetrate, and the use of low voltages will minimize scatter. The x-ray tube current exposure time product was relatively low at 7 mAs. The image was obtained using a 25 cm x 30 cm computed radiography cassette, with the phantom in contact with the imaging plate, and employed the small (0.6 mm) focal spot size. The enlarged image of the line pair phantom indicates that the limiting spatial resolution approaches 3 line pairs per mm. For a imaging plate with a dimension of 25 cm, and an image matrix size of 2,048 (2k), the pixel size is 120 micron; the theoretical limiting spatial resolution is thus 1/( 2 x 0.122) mm, or 4 line pairs per mm which is close to the observed value (~3 lp/mm).
The radiograph shown above was taken using the same techniques (kV/mAs), and the identical (contact) irradiation geometry, but this time employing the large focal spot size of 1.2 mm. The limiting spatial resolution is essentially the same (i.e., ~3 lp/mm) that was achieved using the small focal spot. This example shows that for contact radiography, the size of the focal spot has negligible effect on the spatial resolution performance. The next case, however, shows that this is not the case when radiographic imaging is performed using magnification mode.